1. Field of the Invention
The invention starts from a heat- and creep-resistant steel which has a martensitic microstructure produced by a heat-treatment process, which, in addition to iron and about 8-13% by weight of chromium, contains at least silicon, manganese, nickel, molybdenum, vanadium, niobium and tungsten. Such a steel can be produced by forging or casting or by powder-metallurgical means and, owing to its properties, it can be used with particular advantage for the manufacture of heat- and creep-resistant components of gas- and steam-operated power stations, such as in particular thermal turbo-machines, for example gas turbines or steam turbines or compressors, or steam generators and other high-temperature installations and machines.
Especially in the development of steam turbines, an improvement of the efficiency by raising the temperature and pressure of the fresh steam is the main objective. Thus, a raising of the temperature and pressure from the nowadays usual values of about 550.degree. C. and 240 bar to about 650.degree. C. and 300 bar would improve the thermal efficiency of the steam turbines by about 10%. The associated reduction in fuel consumption not only lowers the production costs of electric power but at the same time considerably reduces the pollution of the environment. At the same time, steam turbines operating at high temperatures and pressures require a high flexibility in operation, such as, in particular, short starting times and a capability for peak load operation. For this purpose, however, a steel having a high strength and high ductility is required. This steel should show a predominantly ferritic and/or martensitic microstructure, since such a steel is substantially less expensive as compared with austenitic steel and, in addition, also has a higher thermal conductivity and a lower thermal expansion, which is particularly important for flexible operation of steam turbines.
2. Discussion of Background
The invention here refers to a state of the art such as results, for example, from DE 3,522,115 A. A martensitic steel known from this state of the art contains, in percent by weight, in addition to iron, 0.05-0.25 of carbon, 0.2-1.0 of silicon, up to 1 of manganese, 0.3-2.0 of nickel, 8.0-13 of chromium, 0.5-2.0 of molybdenum, 0.1 to 0.3 of vanadium, 0.03-0.3 of niobium, 0.01-0.2 of nitrogen and 1.1-2.0 of tungsten. This steel has, at room temperature, an elongation at break of at least 18% and is distinguished at a temperature of up to 600.degree. C. by a high creep strength. At temperatures of 600.degree. C. and higher, however, a high structural stability, a low tendency to embrittlement and especially also a high oxidation resistance are demanded from the steel used, in addition to a high creep strength.